1. Field of the Invention
The present invention relates to a method for measuring the ac specifications of a microprocessor having a clock generator.
2. Description of the Prior Art
Generally, a square wave having a predetermined frequency is adopted as a clock signal used in a digital system for such reasons as that the states of various kinds of signals are binary and that it is convenient in the process of successively generating different signals from one signal. Due to the fact that almost all of the microprocessors that have been put to practical use in recent years are of von Neumann type, such a clock signal is used not only as an operating reference signal for the microprocessor itself, but also as a synchronizing reference signal that is required in inputting or outputting an internal signal of the microprocessor or as a reference signal for synchronizing an input or output signal between an external circuit and the microprocessor. An example of such a microprocessor is found in the 16/8-bit microprocessor .mu.PD70208 (Document Number IC-7078B (third edition), published in May, 1989) made by NEC Corp.
In a prior art microprocessor, a CPU which carries out signal processing, a clock generator and a peripheral circuit located in the periphery of the CPU, such as an auxiliary memory circuit or a signal input/output control circuit intended to enhance the operating speed of the microprocessor, are assembled on the identical chip. A crystal oscillator from outside of the chip is connected to the connection terminals that are joined to the clock generator. The CPU and the peripheral circuit are connected on the identical chip by signal lines for mutually transferring data, address signals and various kinds of signals. The clock generator is connected to a driver within the chip for amplifying an input signal to each control circuit unit within the chip, via a first driver that amplifies a clock signal from a clock oscillator to output it to a control circuit unit, and the output end of the first driver is connected within the chip to a first connecting terminal via a second driver. The output end of the driver within the chip is connected within the chip to the CPU and the peripheral circuit, and the output end of the CPU is connected to a second connecting terminal within the chip via a third driver. The first connecting terminal and the second connecting terminal are respectively connected to an external circuit in the outside of the chip. Accordingly, a clock signal generated in the clock generator is supplied almost simultaneously to the CPU, the peripheral circuit and the external circuit. Each of the CPU, the peripheral circuit and the external circuit executes the internal processing while synchronizing with the clock signal, and the signal generated as a result of this processing is output also synchronizing with the clock signal.
In order to evaluate the performance of the microprocessor its ac specifications are measured. What is meant by the ac specifications are the temporal specifications between the transition points (ac states) of the input/output signals. The contents of the specifications can be classified into the following items.
1. The cycle time of the clock signal PA1 2. The time width of a high level or a low level of the signal PA1 3. The rise time/fall time of the signal PA1 4. The delay time (the time from "completion of transition of a first signal" to the corresponding "completion of transition of a second signal") PA1 5. The setup time (The setup time margin for sampling a first signal with a second signal. It specifies the time from "completion of transition of the first signal" to "starting of transition of the second signal") PA1 6. The hold time (The hold time margin for latching or sampling a first signal with a second signal. It specifies the time from "completion of transition of the second signal" to "starting of transition of the first signal") PA1 7. The float delay time (The delay time in which, for a transition of a first signal, a second signal (output) goes to a floating (high impedance) state. It specifies the time from "completion of transition of the first signal" to "starting of transition of the second signal") PA1 8. The timing chart of the ac specifications (arranging into a chart of the above-mentioned items) PA1 9. The measuring points of the various input/output signals in the ac specification timing chart (for example, the measuring points for a maximum level of 2.4 V and a minimum level of 0.45 V are 2.2 V and 0.8 V, respectively)
The above-mentioned items define the temporal specifications between the transition points of the input/output signals. Considering the fact that these input/output signals are synchronized with the clock signal, it is mandatory that the clock signal and the above-mentioned ac specifications keep a close relationship.
The prior art microprocessor described above includes the following problems.
In taking measurement of the above-mentioned ac specification, the crystal oscillator is disconnected from the connecting terminals to input a clock signal from an IC tester to the connecting terminals, and the ac specification is measured using the first and the second connecting terminals. However, the ac specification measured in this manner does not correspond to the ac specification from the first connecting terminal to the second connecting terminal. Therefore, the reference signals of the ac specifications for the CPU within the chip and the external circuit in the outside of the chip adopted in defining the ac specifications, among the specified electrical properties, of the microprocessor, are not set to be equal.
In order to measure the ac specifications of a chip having a clock output terminal as in this example it becomes necessary to further measure the specification from the clock oscillator that constitutes the clock generator to the first connecting terminal, requiring a large amount of work.
In defining an ac specification of the microprocessor, after measuring the temporal specification of the second connecting terminal with respect to a clock signal to the connecting terminal by using an IC tester, the temporal specification of a clock signal to the second connecting terminal with respect to a clock signal to the first connecting terminal is defined by further measuring the delay time of the clock signal in the first connecting terminal with respect to the input clock signal. As in the above, the measurement of the AC specifications of the microprocessor according to the conventional technology not only generates troubles but also increases the measurement errors.